A critical part of any advanced semiconductor integrated circuit involves the one or more metallization levels used to contact and interconnect the active semiconductor areas, themselves usually residing in a fairly well defined crystalline silicon substrate. Although it is possible to interconnect a few transistors or other semiconductor devices, such as memory capacitors, within the semiconductor level, the increasingly complex topology of multiply connected devices soon necessitates another level of interconnect. Typically, an active silicon layer with transistors and capacitors formed therein is overlaid with a dielectric layer, for example, silicon dioxide. Contact holes are etched through the dielectric layer to particular contacting areas of the silicon devices. A metal is filled into the contact holes and is also deposited on top of the dielectric layer to form horizontal interconnects between the silicon contacts and other electrical points. Such a process is referred to as metallization.
A single level of metallization may suffice for simple integrated circuits of small capacity. However, dense memory chips and especially complex logic devices require additional levels of metallization since a single level does not provide the required level of interconnection between active areas. Additional metallization levels are achieved by depositing over the previous metallized horizontal interconnects another level of dielectric and repeating the process of etching holes, now called vias, through the dielectric, filling the vias and overlaying the added dielectric layer with a metal, and defining the metal above the added dielectric as an additional wiring layer. Very advanced logic device, for example, fifth-generation microprocessors, have five or more levels of metallization.
Conventionally, the metallized layers have been composed of aluminum or aluminum-based alloys additionally comprising at most a few percent of alloying elements such as copper and silicon. The metallization deposition has typically been accomplished by physical vapor deposition (PVD), also known as sputtering. A conventional PVD reactor 10 is illustrated schematically in cross section in FIG. 1, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor 10 includes a vacuum chamber 12 sealed to a PVD target 14 composed of the material to be sputter deposited on a wafer 16 held on a heater pedestal 18. A shield 20 held within the chamber protects the chamber wall 12 from the sputtered material and provides the anode grounding plane. A selectable DC power supply 22 biases the target negatively to about -600 VDC with respect to the shield 20. Conventionally, the pedestal 18 and hence the wafer 16 is left electrically floating.
A gas source 24 of sputtering working gas, typically chemically inactive argon, supplies the working gas to the chamber through a mass flow controller 26. A vacuum system 28 maintains the chamber at a low pressure. Although the chamber can be pumped to a base pressure of about 10.sup.-7 Torr or even lower, the pressure of the working gas is typically kept between about 1 and 1000 mTorr. A computer-based controller 30 controls the reactor including the DC power supply 22 and the mass flow controller 26.
When the argon is admitted into the chamber, the DC voltage ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target 14. The ions strike the target 14 at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target 14. Some of the target particles strike the wafer 16 and are thereby deposited on it, thereby forming a film of the target material.
To provide efficient sputtering, a magnetron 32 is positioned in back of the target 14. It has opposed magnets 34, 36 creating a magnetic field within the chamber in the neighborhood of the magnets 34, 36. The magnetic field traps electrons, and for charge neutrality, the ion density also increases to form a high-density plasma region 38 within the chamber adjacent to the magnetron 32. However, it is understood that a plasma of decreasing density extends towards the wafer 16.
With the continuing miniaturization of integrated circuits, the demands upon the metallization have increased. Many now believe that aluminum metallization should be replaced by copper metallization. Murarka et al. provide a comprehensive review article on copper metallization in "Copper metallization for ULSI and beyond," Critical Reviews in Solid State and Materials Science, vol. 10, no. 2, 1995, pp. 87-124. Copper offers a number of advantages. Its bulk resistivity is less than that of aluminum, 1.67.mu..OMEGA.-cm vs. 2.7.mu..OMEGA.-cm for pure material, and any reduction in resistivity offers significant advantages as the widths and thicknesses of the metallization interconnects continue to decrease. Furthermore, a continuing problem with aluminum metallization is the tendency of aluminum atoms in an aluminum interconnect carrying a high current density to migrate along the interconnect, especially away from hot spots, in a process called electromigration. Any excessive amount of such migration will break an aluminum interconnect and rendering inoperable the integrated circuit. Copper-based alloys exhibit significantly reduced levels of electromigration.
Copper metallization is an unproven technology and is acknowledged to offer difficulties not experienced with the conventional aluminum metallization. However, it may afford ways to circumvent problems inherent in aluminum metallization.
Murarka et al. in the aforecited review article recommend alloying copper with magnesium or aluminum to improve the interfacial qualities. Later work done by the Murarka group at Rensselaer Polytechnic Institute and their collaborators have developed a useful technique for forming dependable copper interconnects and provide a model for its operation. As Lanford et al. describe in "Low-temperature passivation of copper by doping with Al or Mg," Thin Solid Films, vol. 262, 1995, pp. 234-241, sputtering is used, as illustrated in the schematic cross section of FIG. 2, to deposit a film of copper alloy on a substrate. The primary examples of the alloying element are aluminum and magnesium. The copper alloy film can be deposited as alternating layers of copper and the alloying element, or the two constituents can be co-sputtered, for example, by use of a copper alloy sputtering target. After completion of the sputtering at near to room temperature, the wafer is annealed, for example, at 400.degree. C. in argon for 30 minutes. The annealing causes a large fraction of the magnesium to diffuse to the outside of a remaining copper film and to react with any oxygen present at the interfaces to form a film of magnesium oxide. The MgO film encapsulates the Mg-alloyed Cu body. The upper free surface of the copper film is passivated by the MgO film. Magnesium oxide is a stable oxide and stops growing at a thickness in the range of 5 to 7 nm. The thin oxide is not believed to cause a high contact resistance, but in any case the oxide can be removed by a sputter etch prior to the deposition of a subsequent metallization. Lanford et al., ibid., suggest that the free surface is oxidized to MgO by oxygen impurities in the argon.
Metallization in advanced integrated circuits faces a demanding requirement in filling high-aspect ratio holes. Increasing device density requires that the feature sizes be further reduced. However, dielectric breakdown has prevented the thickness of interlevel dielectric levels from being similarly reduced. As a result, the aspect ratio of vias and contacts has been increasing. The aspect ratio is the ratio of the depth of the hole through the dielectric forming the via or contact and the minimum lateral size of that hole. An aspect ratio of 5:1 is considered developmental technology, but even higher values will be required. Assuming that high aspect-ratio holes can be etched, the problem remains of filling them with metal for the interlevel connection. The geometry of high aspect-ratio holes is unfavorable for sputtering since conventional sputtering is fairly isotropic so that little sputtered material strikes the bottom of the hole compared to the lip of the hole, and the sputtering is likely to bridge the top of the hole and prevent any further deposition. The hole filling problem is illustrated in cross section in FIG. 2. A narrow and deep hole 40 is etched into a silicon oxide substrate 42, which contains unillustrated structure to be electrically contacted. A copper layer 44 is then filled into the hole 40. If the filling is performed by a standard PVD process including an initial cold deposition to form a surface layer followed by a hot final deposition to complete the filling, a void 46 is likely to form in the hole because the copper dewets from the oxide sides of the hole 40. Once the copper bridges over the void 46 midway through the deposition, it is virtually impossible to remove the void and complete the hole filling.
At least two techniques are used to overcome the unfavorable geometry of hole filling, directional sputtering and reflow. In directional sputtering, one or more of various techniques are used to produce a flux of sputtered particles incident upon the wafer which are heavily concentrated in the normal direction. Reflow relies on the fact that metals flow at moderately low temperatures so that the metal, although initially deposited in a undesirable distribution, is made to flow into the hole and to fill it. The reflow may be produced in a post-deposition anneal or may occur on an ongoing basis during a hot deposition.
Directional sputtering may be achieved by many methods, including long-throw, collimation, and electrostatic attraction of ionized sputtered ions in a high-density plasma. Directional sputtering, although not required by the invention, is advantageously used in conjunction with it, as will be discussed later.
Reflow of metallizations, especially copper, presents several difficulties. Unlike aluminum, copper has a relatively high melting point. Heating the substrate to the melting temperature of the metallization would incur too high a thermal budget and may be inconsistent with prior processing steps. Aluminum and copper do flow at somewhat lower temperatures than their melting points, but the interface between either of these metals and the silicon dioxide forming the usual interlevel dielectric is not favorable for reflow. Neither aluminum nor copper wets well with silicon dioxide at certain high temperatures. As a result, these metals do not flow in a smooth layer down a wall of silicon dioxide. Indeed if aluminum or metal is present as a thin layer on a surface of silicon dioxide, the metal tends to ball up in isolated locations.
Xu et al. have addressed the reflow problem with aluminum metallization in U.S. patent application Ser. No. 08/628,835, filed Apr. 5, 1996. They recommend using a carrier layer of TiN and possibly Ti deposited by a high-density plasma to perform a number of functions including increasing the adhesion of aluminum deposited in a narrow aperture extending through silicon dioxide. The carrier layer acts as a glue layer that adheres well to silicon dioxide and also acts as a wetting layer for the later deposited aluminum. As a result, the aluminum flows down the carrier layer at a relatively low temperature and thus easily fills the hole.